The invention relates to a process for the manufacture of injection-molded articles, especially toothbrush bodies or toothbrushes, wherein the molding material is conveyed from a supply container by means of an injection unit, in particular a screw-piston extruder or a similar material delivery means to the mold cavities. Beyond this, the invention relates to an injection molding machine for the carrying out of this process.
Toothbrushes, particularly in regard to the toothbrush body, have been produced in a multiplicity of colors, generally limited to five different colors. Production in these colors could be executed by the installation of a corresponding number of injection molding machines. However, setting aside the extreme cost factor, such a measure had disadvantages, in that as a rule, different quantities for the toothbrush body lots for each color resulted in no uniform production quota for each machine. Instead, in practice, color changes were undertaken during the production of toothbrush bodies. This again proved to be a complicated matter, very time consuming and caused a poor result in production, which was seen as anything but trivial.
The various coloring substances were, in one method, mixed dry into the plastic granulate at hand for the toothbrush bodies, and this mixture was then heated in the screw-piston extruder, or a similar delivery system, then liquified and apportioned through a mold-entry and internal cavity connection system for distribution into the individual mold cavities or nests thereof, while the injection procedure was in action. In another method, liquid coloring material was employed, which if a screw-piston extruder was being used, was fed into the entry chamber thereof.
If a color change is to be made, then all traces of previous color must be purged from the screw extruder, as well as the delivery channels. This operation is carried out, so that the new color is brought into the system, without stopping the molding of toothbrush bodies until the last remnants of the previous color have been eradicated. This can take as much as an hour and the toothbrush bodies so made, form an unusable product, since they show both the old and the new coloring. Furthermore, an operator""s attention is required to monitor the color change of an otherwise automatically running injection molding machine.
The ensuing costs for such a color change are so great, that this procedure is seldom followed as a matter of practice. On this account, toothbrush bodies of respectively one color are produced on an inventory basis. Where production exceeding the current production needs for toothbrush bodies of one color is concerned, large quantities of toothbrush bodies must therefore be placed in interim inventory, so that, for the subsequent insertion of the bristles, toothbrush bodies of all possible colors are made available in the desired color assortment ratios as called for. For this operation, a corresponding, dedicated area in the factory is necessary. It is also problematic that measures must be taken to avoid, that over a lengthy storage time, a contamination of the toothbrush bodies occurs. This is valid especially in the production of toothbrush bodies from the standpoint of hygienic demands.
In order to avoid this interim inventory storage, it is certainly possible to forward the produced toothbrush bodies immediately to the insertion of the bristles. However, then an even more circumstantial color sorting following either the insertion operation or the packing could be required.
Thus, the object of the present invention is to create a process, in which, according to immediate need, a change in production to a different variety, at least in regard to the coloring of the injection molded articles, can be carried out in a short time. Also the possibility should be realized, of simultaneously manufacturing different injection molded items with one injection molding machine. Finally, an injection molding machine for carrying out the process is to be provided.
For the achievement of this object, insofar as it concerns the process and in accordance with the invention for the manufacture of injection molded items, the molding material is to be combined with at least one additive which is fed into the distribution path of the molding material as this is being delivered to one or more mold cavities. The introduction of the additive is to be at a point in the direction of flow following the injection unit, behind its entry point, and the additive material and the molding material are to be mixed.
In this way, it becomes possible, with a single injection molding machine, to simultaneously produce different items of injection molding manufacture. This capability substantially reduces the cost and, in many applications, a changeover of the injection molding machine from one item to another becomes a matter of little time and trouble. The basic material issuing from the injection unit on its way to the single or grouped nests of molds receives one or more additives, so that, in accordance with each additive, different items can be simultaneously produced.
As an example, in the case of an injection molding die with, customarily, a multiplicity of mold cavities, it is possible, by the above described advantage, that toothbrush bodies differing in colors can be injection molded entirely on an as needed basis. Then in accordance with the required assortment, these bodies can be suitably color grouped as produced in the injection molding machine.
Additionally, in the case of a possibly foreseen changeover in the production of injection molded items to a different design, such items which may carry the previous additive are essentially smaller and hence a quicker changeover becomes possible. At least the screw-piston type extruder or similar delivery means operation, is isolated from that channel section which contains the additive, for instance coloring material, so the molding material found in the extruder need not purged. With each new position of the entry point of the color, or the like additive, within the transport path of the molding material as it approaches the mold cavities, the zone containing such additive or coloration is less and thereby, for an individual color change, the requirement for purging also is reduced. By means of mixing the molding material and the additive just after their being joined, the feed of the mix can be done even closer to the respective mold cavity.
If a multiplicity additive feed points are foreseen, it is possible that these can be conducted directly into the mold entry opening by means of one feed line. In this case, practically from injection cycle to injection cycle, a change of color could be undertaken, without the necessity of an intermediate purge procedure. Without additional measures for the thorough mixing of color and molding material, it is possible, by direct injection or injection of the color near to the respective mold openings, to bring about desired, colorized formulations, whereby the toothbrush body has no uniform, penetrating color, but exhibits a color pattern. The chromatic or optical appearance can be obtained through the input of several colors or by the addition of color varied in combination with other additives.
Besides colors, other additive materials, possibly in combinations, can also be added, after the injection unit with respect to the direction of the flow to the molding material within the transport distribution system to the molds. Again, this additive input to the molding material can be done directly at the mold entry openings. Advantageously, additives are available as colors, preferably liquid colors, powders, metal platelets, reinforcing or supportive material, deformation preventatives, chemically active additives, such as, for instance, a propelling agent, hardening material, softener, or the like.
Metal platelets can provide a scintillating effect whereby the injection molded product can be varied in its visible appearance, possibly in connection with different colors. As a reinforcing or supporting substance, fiber material. such as glass fibers or carbon fibers can be used, and besides these crushed stone, talcum, or the like can be employed. Aids for extraction of products from molds serve for a better protection for the items made by injection molding, for instance, silicone can be used for this purpose.
Chemically active additives can be employed for the altering of the properties of the injection material, for instance, to have a desirable effect on the consistency of the material to be molded. Such an additive can also be an activator, which would accelerate the solidification of the molded material. A softener can be added, especially when parts of a toothbrush body are to be provided with an enveloping spray of a rubbery elastic material. An accelerator means would be used, if the injected material comprised two substances.
In this way, besides reducing time and costs, further advantages emerge from a change in production from one design to another of injected molded articles and to expanded applications of the items.
The above named or comparable additives, in accordance with the inventive process, permit themselves to be added generally in close proximity to the mold cavities, since they become practically immediately effective. Due to this, making the addition in the supply container for the molding material which entails a long transport path to the mold cavities, or molds, becomes superfluous.
In a plurality of mold cavities provided in molding dies, these may be connected group-wise by a main channel distributor, or several channel subdistributors to the delivery system for the material to be injected. In this case, the possibility arises, in one embodiment of the invention, that the additive material at least can be supplied in its own channel distribution and subdistribution system.
If color, in the form of an additive, is input in a main distributor line, then all connected cavities of the molding dies are thus supplied with uniformly colored injection material. Upon the feed of the color into one or more subdistributors, then it is possible, even within the molding plate, to manufacture group-wise, differently colored toothbrush bodies.
Further the possibility exists, of inserting the additive directly into one or more nozzles, which nozzles are connected to the mold cavity. In this case, in proportion to the number of the available mold cavities, or groups thereof, and dependent upon the number of the desired colors, color changes are seldom necessary, except at considerably extended time periods, and indeed, under certain circumstances, no color change operation may be again necessary. In this way, the problems connected with color change operations do not occur again, or are substantially reduced in scope.
In accordance with a further development of the invention, the additive is given to the injection material in dosages. By the addition of differing additive quantities, the mixing proportions, especially where color (for instance, white material) is concerned, can be varied. Furthermore, by this method, within a given color, different hues can be achieved. The dosage can be carried out by periodic variation of the additive input and/or by changing the pressure of the additive delivery pressure.
The invention further relates to an injection molding machine for carrying out the inventive process. This injection molding machine includes an injection molding die, or dies, which contains an injection mold. The machine also includes an injection unit, in particular with a screw-piston extruder, or similar means of material transport. The function of the extruder is to transport the material to be molded from the injection unit to the distribution or feed channels which lead to the mold cavities. This injection molding machine is characterized, in that at one or more of the distribution channels are connected to one or more feeds for at least one additive. The machine is further characterized in that a respective distribution channel furnished with an additive feed, includes at least one mixing apparatus located, after (with respect to the direction of material flow) its point of connection to the additive feed.
The advantages essentially achieved with this injection molding machine have already been outlined in connection with the advantages of the above described process. Especially, by the measures available with the inventive injection molding machine, changeovers, for instance color alterations, can be carried out for different embodiments of the injection molded items with essentially less expense and in less time. In the case of injection molding dies containing several groups of mold cavities, as is customary in the manufacture of small brush bodies, especially toothbrush bodies, it is possible that simultaneously, different, especially toothbrush bodies of varied colors, can be produced.
Further, in a molding die where a multiplicity of mold cavities or groups of mold cavities are present, corresponding to the optional colors or embodiments of the brush bodies, a changeover of said injection molding machine, in the customary concept, becomes superfluous. In addition to this, the characteristics of the injection molded items can be altered by various additives and/or the manufacture improved, in fact, made less complicated. The mixing apparatus serves for a through mixing of the additive material into the basic material to be molded. Also, a good mix can be achieved by means of mixing at the feed point of the additive material close to the mold cavity. Such a mixing chamber has the advantage, that simple construction therefor is available and with no disturbing downtime.
A simple embodiment of a mixing apparatus can be provided by a change in the cross-section of a channel, preferably by a mixing chamber formed by an enlargement of the cross-section. Such a mixing chamber has the advantage, that a simple construction is employed and no disturbances can be attributed to the device.
In order to additionally improve the thorough mixing of additive and injection material, the entry point of the additive feed at the combined feed channel can be designed as an annular type nozzle. By this means, the additive material penetrates the injection material through a multiplicity of perforations, spaced at circumferentially apportioned distances about the additive line termination, thus enabling an intensive mixing at this location. Especially in a combination of one or more successive mixing apparatuses, or, for instance, a mixing chamber, it is possible to obtain a good, intimate mixing of additive and injection material.
Another advantageous embodiment is so designed that provision is made for a hot distribution channel system for the handling of melted injection material (hereinafter xe2x80x9cmeltxe2x80x9d), the melt, before it is introduced into the mold cavity, is mixed with at least one additive, the distribution system includes at least one melt channel in which at least one feed line section for the entry of additive material opens, provision is made for at least one mixer connected in the direction of material flow, and the mixer is designed as a mixing assembly, which incorporates at least two mixer element sections, the axes of which are not in common alignment.
The advantage of this embodiment, wherein the design of the mixer is as at least a two-part unit and not as a single straight-line mixing section, lies in that it is now possible to have an extremely flexible, arrangement of the mixer which adapts itself to the geometry of the hot distribution channel.
The design of the hot channel distributor as a main distributor brings with it the advantage, that different subdistributors connected to the main distributor can be supplied with melt which is mixed with various additives. For this purpose, a line for one or more additives, which connects into a length of the injection material feed line, and also a mixer in the direction of material flow, are both placed behind a branching leading to one or more subdistributors. In this manner, single subdistributors or groups thereof can be supplied with some flexibility to body melts which are mixed with various additives.
An even more flexible supply method is possible, if the hot distributor is designed as a subdistributor connected with several hot runner nozzles. With the aid of such a hot channel distributor, individual groups of hot runner nozzles can advantageously be supplied with melts, which melts are already mixed with various additive materials. If more mixers are employed then, advantageously, the length and the cross section of the single mixing stretches are equal. By this means, a balanced flow relationship is achieved, which, in turn, results in a uniform filling and subsequent repressure for each mold cavity.
In a further advantageous embodiment of the invention, the exit opening of a first section is connected in the direction of the material flow to the intake opening of a second section of the two mixing sections by means of a turn-around fitting. By this configuration, the goal is attained, that the first section provides a flowing material passage in one direction, which is essentially opposite to the flow direction in the second section. The turn around fitting, in this embodiment, allows the non-straight through configuration of the two sections to be realized. By an appropriate angular choice of the turn-around fitting, both sections accommodate themselves to the shape of the distributor and other components present, such as heating wires, or shutoff valves of the hot runner nozzles can be given consideration of proper placement. If the reverse flow angle of the fitting is 180xc2x0; then the length of the mixing section is halved by a loop, so that both channels of the mixing sections are disposed parallel to one another. In this way, an extremely compact hot distributing channel can be designed, which provides a mixer of high capacity, demanding a minimum of space. In another advantageous embodiment, the hot distributor channel can be constructed, using two horizontal planes located one above the other, whereby the first section is found in the lower and first plane and the second section is located in the upper and second plane. This symmetrical arrangement of the two parallel sectional lengths aids in the formation of a balanced channel layout, with equally long flow paths.
The hot distributor channel can be seen as divided by the two planes, into lower, middle and upper layers, whereby in each layer, grooves are provided and after the soldering of the said layers by vacuum diffusion process, the grooves of respectively matching layers, now form the upper and lower halves of a melt channel.
Experience has shown it to be of advantage, if the two mix sections are designed as two static mixers, which can effect a high degree of mixing of the melt in a minimum of space. Such static mixers are known from the catalog of Mixing and Reaction Technology (1994) of Sulzer Chemtech, GmbH specifically for installation in injection molding machines. These known mixers, however, should only complete the homogenization of the melt with color(s) in addition to a mixing of said melt by the action of an extruder.
In order the simplify the installation of the static mixer in the hot channel distributor, advantageously, the two static mixer elements can be soldered directly into the melt channel. For this purpose, prior to the soldering of the individual layers, each of the static mixing elements is inserted in one of the melt channel halves, which said half is found in each of the above described layers.
Preferred static mixer elements encompass respectively, a plurality of screw-shaped, twisted deflection plates, which, in alternate fashion, are arranged sequentially in a left and right sense. Each deflection plate includes two standing edges, perpendicular to the direction of flow, which, respectively, are bound to the adjacent edge of the neighboring deflection plate and are turned, in relation to the neighboring deflection plate, at a specified angle. Such static mixer elements divide the melt and mix non-homogenous streams again with each other. A particularly good dividing effect and mixing action is achieved, when the edges bound to one another are turned to an angle of 90xc2x0 with each other. The adjacent edges of neighboring deflection plates can be connected by spot welding to one another.
In a section of a hot feed channel, an end piece of the additive feed can project into and coaxial with the melt channel, whereby an annular opening between the outer wall of the said end piece and the inner wall of the melt channel is formed for the flow of the melt. This advantage allows that the additive be guided into the center stream of the melt flow, which is a favorable situation for the desired uniform mixing of the melt with the additive. The diameter of the end piece of the additive channel can be less than the diameter of that part of the additive channel onto which the said end piece has been attached. This has the favorable effect, that the melt is fortunately hindered from entering into the additive end piece. The additive channel can be connected to a reservoir and a positive pressure pump. The pump impels the additive to be mixed from the reservoir into the end piece of the additive line with an appropriate pressure. The placement of the reservoir and the high pressure pump outside of the hot distributor channel enables a simple maintenance operation as well as a quick and simple additive material switch. Advantageously, the pressure achievable by the high pressure additive pump has a proper relationship to the existing pressure in the melt channel for the injection nozzle.
The end piece of the additive feed can be closed for metered dosage of the additive material by a needle valve, which, by means of a solenoid or magnetic valve is controllable for intermittent operation. With the aid of the needle valve and of the solenoid, a finely tuned dosage of the additive is possible.
It has shown itself as advantageous, if the solenoid of the needle valve is activated at a frequency of 30 to 100 cycles per second and the thrust of the needle valve is limited to 0.1 to 0.01 mm.